This invention relates to printed circuit boards and methods of fabricating them. The printed circuit boards have high circuit and component density, including either or both of surface mount components or additional circuitry layers directly atop plated through holes. The printed circuit boards of the invention are especially useful where the density of plated through holes required to service the I/O""s of the surface mount devices is such that there is no surface area available for attachment pads interstitial to the plated through hole grid. The printed circuit boards of the invention are useful with fine pitch Ball Grid Array (BGA) integrated circuit modules and flip chip attach integrated circuit chips.
The printed circuit boards are prepared by first laminating a subcomposite with exterior sheets. Then a first pattern of holes are drilled through the subcomposite for plated through holes. This is followed by plating over the exposed surfaces of the subcomposite and the plated through hole barrels to form plated through holes. Fill material is forced through the drilled exterior sheets and into the plated through hole to fill the plated through holes with plated through hole filler composition. The subcomposite is then drilled to form a second pattern of through holes that are to remain unfilled. The through holes of this second pattern are plated, and a desired circuit pattern of surface circuitization is formed.
Increasing levels of integration of integrated circuit chips reduces the chip count of a functional circuit, while significantly increasing the I/O count of the individual integrated circuits making up the functional circuit. This drive for increased circuit and component density in the individual integrated circuit chips leads to a parallel drive for increased circuit and component density in the printed circuit boards carrying the chips and in the assemblies using them.
The increased circuit and component density in the printed circuit boards makes the ability to locate either solder surface mount components or place additional circuitry layers directly above plated through holes highly desirable. This is especially the case when the density of the plated through holes required to service the I/O""s of the surface mount components is such that there is no surface area available for attachment pads interstitial to the plated through hole grid.
The problem is especially severe with fine pitch ball grid array components (BGA) and flip chip attach integrated circuits. Soldering of these surface mount components to the surface pads, i.e., lands, of conventional plated through holes is highly undesirable. This is because the solder used for assembly tends to wick down into the plated through holes. The result is low volume, unreliable solder joints.
One solution that has been proposed is filling the plated through holes. However, known methods of filling plated through holes of printed circuit boards have deficiencies. For example, they suffer from bleed of the resin component of the PTH fill material along the surface of the boards. This resin also bleeds into holes which are not to be filled. This leads to short circuits and to soldering defects during assembly.
It is a primary object of the invention to facilitate the connection of high I/O density integrated circuit chips to highly packed printed circuit boards and cards.
It is a further object of the invention to provide for high interconnection escape.
It is a still further object of the invention to provide a process to make high function density printed circuit boards having selectively filled through holes.
These and other objects of the invention are achieved by the printed circuit board of our invention. The printed circuit board is one having two types of plated through holes. The first type of plated through holes extend to and through an exterior surface of the printed circuit board for receipt of a pin-in-through-hole module or component. The second type of plated through holes terminate at or below the exterior surface of the printed circuit board and allow for further surface circuitization as well as surface mount attachment. These plated through holes contain a fill composition, for example, an electrically or thermally conductive fill composition.
According to a further embodiment of our invention there is provided one or more plated through hole that terminate in a surface mount contact. Typically the plated through holes that terminate in a surface mount contact have a conductive cap bonded to the surface mount contact.
According to a still further embodiment of our invention there is provided at least one of the plated through holes terminating at a layer of dielectric material; that is, extending to but not through the layer of dielectric material. This allows external circuitization on the dielectric that overlays the plated through hole.
According to our invention the filler is a composition of an organic polymeric material, optionally with a particulate filler added thereto to modify the thermal or electrical conductivity or the coefficient of thermal expansion. In at least one of the second plurality of plated through holes the filler composition may be a thermally or electrically conductive fill composition. The filler composition is compounded to have a coefficient of thermal expansion matched to the coefficient of thermal expansion of the printed circuit board substrate. The fill composition includes as components a resin, also referred to herein as a binder, and a particulate, conductive or non-conductive filler component.